TROPICAL TROPOSPHERIC OZONE FROM TOTAL OZONE MAPPING SPECTROMETER BY A MODIFIED RESIDUAL METHOD

A modified residual method for deriving time-averaged tropical troposp heric ozone (TTO) from total ozone mapping spectrometer (TOMS) has bee n refined [cf Kim et al., 1996] and applied to a 2-year period, 1991-1 992. This period was selected because of the availability of a unique sonde record at tropical Atlantic and near-Atlantic sites: Natal, Braz il; Ascension Island; and Brazzaville. The essentials of the modified- residual method are (1) Fourier analysis to identify the latitude rang e for use of the method; (2) use of the wave-like pattern to estimate stratospheric ozone column and a background tropospheric ozone column, which is defined as the amount of ozone expected in the absence of ch emical pollution (3) assumption that the wave-one feature is troposphe ric; and (4) the stratospheric and background tropospheric ozone amoun ts consistent with the constraint of the three station ozonesonde meas urements for total tropospheric ozone. An excess ozone (pollution) sig nal is also derived from TOMS total ozone. Parameters derived from the modified residual method are evaluated with independent data to verif y the quality of the TTO. Derived tropical stratospheric column ozone for 1991-1992 agrees very well with the corresponding UARS Microwave L imb Sounder and Stratospheric Aerosol and Gas Experiment records (to w ithin 5 Dobson units (DU) for the tatter). Background tropospheric ozo ne, which exhibits a seasonality in 1991-1992, has a 24 +/- 4.8 DU mea n value at wave maximum (similar to 0 degrees longitude) and a 15.2 +/ - 6.0 DU mean value at 180 degrees longitude. The Atlantic value is re markably similar to the climatological 26 DU previously inferred from subtropical sondes [Kim et al., 1996; Komhyr et al., 1989]. The simila r to 5 DU standard deviation is the accuracy of the modified-residual method and is the best that can be achieved given the 15-day averaging of the sondes (1 sigma = 5.3 DU). The wave-like pattern in 1991-1992 has a semiannual periodicity with zero wave amplitude in June and Dece mber.